Modern building structures often make use of barrier membrane sheet materials to control the flow of moisture in and out of a building and also to control the ventilation or movement of air through a wall or roof structure for example.
Typical membrane products are designed to be weather resistant, keeping out liquid water and resisting wind pressure. At the same time however it is highly desirable that the membrane be formed so that water vapour may pass through relatively freely to avoid problems of dampness or condensation within a building or within a wall or roof structure. A number of water vapour permeable membrane products are available and combine weather resistance with water vapour permeability. One type of product, incorporating macroporous film technology, has small pores which allow diffusion of water molecules, driven by vapour pressure differential, from one side of the membrane sheet to the other (generally from inside the structure to out). Membranes which allow passage of water vapour but not air are generally referred to as vapour permeable air barriers.
For some applications a membrane sheet that allows passage of air as well as of water vapour is desirable.
An air permeable membrane allows controlled ventilation and is at the same time highly vapour permeable. Water vapour is readily transported by air movements through the membrane.
Air barrier and air permeable weather resistant membrane sheets find use in a wide range of applications. For example in forming a building ‘envelope’ underneath exterior cladding or roofing material which resists liquid water (rain) but which allows escape of water vapour or trapped moisture from inside the structure.
Building regulations vary from one country to another and the types of membrane used are often dictated by the climate and by local building requirements. In Canada for example, the use of membranes which provide an air barrier are mandated.
Typical membrane sheeting is attached to a wall or roof structure, (for example, sheathing panels of a wall structure) by means of mechanical fasteners such screws or nails or by use of an adhesive for example a bitumen-based adhesive, which is not air or water vapour permeable. These methods have certain disadvantages however. With mechanical fasteners attachment is discontinuous and is only at points where the fixings connect the membrane sheeting to the rest of the structure. This leads to the possibility of air or water passing along underneath the membrane sheet reducing control of the ventilation and humidity. Furthermore mechanical fixings which puncture the membrane (nails or screws) reduce the integrity of the envelope and can provide opportunities for tearing the membrane; during installation; or when the installed membrane is subjected to high air pressures during bad weather. Where an adhesive such as a bitumen-based adhesive is employed the area covered by the adhesive does not permit transmission of water vapour or air. Furthermore a primer is often required to securely attach such adhesives to substrates.
Some of these problems are addressed in U.S. Pat. No. 6,901,712, which provides an air barrier (water vapour permeable) membrane which has a discontinuous adhesive layer attached to one surface. The discontinuous adhesive layer is used to stick the membrane to panels or sheathing substrates of a wall or roof structure without mechanical fixings being required. Self adhesive membrane systems are commonly described as ‘peel and stick’ systems. However the adhesive employed in the discontinuous layer is not itself air or water vapour permeable. This limits the water permeability of the membrane sheet to those parts of the sheet without attached adhesive.
It is an object of the present invention to avoid or reduce one or more of the foregoing disadvantages.